This invention relates to an overlay proofing film having a base film with improved transparency. In the graphic arts, it is desirable to produce a four or more color proof to assist a printer in correcting a set of color prints prior to using them to produce color plates and also to reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplication of the half tone, and should neither gain nor lose color. Visual examination of a color proof should show the following characteristics:
1. Defects on the negative.
2. The best color rendition to be expected from press printing of the material.
3. The correct gradation of all colors and whether grays are neutral.
4. The need, if any, for subduing one of the colors and/or giving directions for altering the film negatives before making the printing plates.
A method of transferring colored images has been applied to the preparation of a color proofing sheet in multicolor printing. Thus, color proofing sheets, for multi-colored printing, have heretofore been made by using a printing press or a proof press while taking all the steps necessary for actual multicolor printing, but such a conventional method of color proofing has been costly and time consuming.
Photographic processes have also been used, especially photographic processes using photopolymers. The usual type of photographic color proofing method is the overlay type.
In the overlay type of color proofing method, an independent transparent plastic support is used for producing a print of each color separation film by applying a photosensitive solution of the corresponding color, and a plurality of such supports carrying prints of corresponding colors are then superimposed upon each other to produce a color proofing sheet. The overlay type of color proofing method has the disadvantage that the grayness and density of the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press or a proof press. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or three colors in register.
As mentioned above, the typical proof sheet adapted for imagewise color exposure consists in the most elementary form of a transparent base sheet coated on one side with a light sensitive composition containing a dyestuff. Most commonly with negative acting materials, the light sensitive composition may be composed of a diazo material, used either alone or in combination with a resinous binder, which composition is caused to harden by exposure to a source of actinic light. The hardening occurs as the result of a photopolymerization, condensation, or coupling reaction which renders the light struck areas insoluble in common developer solutions such as aqueous developers, while the non-light struck areas remain substantially chemically unaltered and soluble. Once developed, the light struck areas of the coating exposed through a negative transparency remain adhered to the base sheet in the form of an image.
In order to overcome the above-stated darkening problem, it is desired that a proofing film be obtained wherein the base film has improved clarity and transparency.